This specification relates to the selection of content items.
Information, such as text, images, and videos, may be displayed in a window of a display screen. In some cases, all of the relevant information is visible in the window. In other cases, however, only a portion of the relevant information is visible in the window. For example, only a portion of the information may be visible in the window when the viewable size of the window is too small to display all of the information in a manner that can be easily viewed and manipulated by a user. To view a portion of the information that is not visible, the window may be scrolled to bring the information into view. Scrolling is the act of sliding the information vertically or horizontally relative to the window. Scrolling does not change the layout of the information, but moves (e.g., pans) a user's view across the information by bringing different portions of the information into the viewing area of the window. A user may scroll the information using touchscreen gestures, mouse movements, or keyboard presses.
Information that is presently visible in a window (i.e., without the need for scrolling to be viewed) may be referred to as above-the-fold (ATF) information. Information that can be viewed only by scrolling the information in the window may be referred to as below-the-fold (BTF) information. Providers of content may present certain information, for example, information that is high in rank, priority, relevance, significance, or importance, above the fold since ATF information is visibly prominent without the need for scrolling. For example, advertisers may prefer to present advertisements in the area that is above the fold because such advertisements would be visible to a user without scrolling.